Third-order Nonlinear Distortion Research Articles

Analysis of nonlinear distortions of DpHEMT structures based on the GaAs/InGaAs compound with double-sided delta-doping

Computational and experimental studies of the characteristics of a high-power AlGaAs/InGaAs/GaAs DpHEMT were performed. A self-consistent numerical solution of the Schrodinger and Poisson equations was used to calculate the band diagram and the electron concentration in the channel of the transistor under study. The electron mobility in the transistor channel was estimated experimentally at 9300 cm2/V·s. The obtained transfer current--voltage characteristic of the transistor was used to calculate the parameters of a model differential amplifier (small-signal gain and third-order non-linear distortion factor). Keywords: AlGaAs/InGaAs/GaAs DpHEMT, nonlinear distortion, spacer layers.

Transmitters for Combined Radio Over a Fiber and Outdoor Millimeter-Wave System at 25 GHz

In the modern wireless networks, millimeter-wave radio-frequency (RF) bands are becoming more attractive as they provide larger bandwidth and higher data rates than the today-used systems operating at frequencies below 6 GHz. In addition, according to the fact that coaxial cables exhibit extremely high attenuation for millimeter-wave RF signals, analog radio over fiber techniques (RoF) form a promising technology for delivering unaltered radio waveform to a remote antenna. This paper experimentally analyzes three types of RoF modulations, namely a directly modulated laser, an electro-absorption modulator, and a Mach-Zehnder Modulator. The primary focus is on the implementation of each RoF transmitter in an RoF system, such as those in 5G networks. The experimental study includes a detailed characterization of an RoF system with a 50-m long outdoor free-space RF channel operating in the frequency band of 25 GHz. Frequency response (S-parameters) and third-order nonlinear distortion are investigated in detail. Tests of EVM performance were conducted using an orthogonal frequency division multiplexing signal modulated with 16-quadrature amplitude modulation (16-QAM) with a long-term evolution signal. It is demonstrated that the transmitters studied can operate under a 13.5% EVM limit given for 16-QAM. Apart from the detailed system performance, the considerable power fluctuations in the 25 GHz free-space RF outdoor channel are reported.

Joint Mitigation of Nonlinear RF and Baseband Distortions in Wideband Direct-Conversion Receivers

Software-defined radio technology is experiencing more and more attention in modern communication and radar systems. The main practical challenge in deploying such technology is related to achieving sufficient linearity and spurious-free dynamic range in the RF front-end, especially in low-cost mass-product devices. This paper focuses on the analysis and digital mitigation of nonlinear distortion in software-define radio devices, building on wideband multicarrier/multiradio direct-conversion receiver principle where a wide collection of radio frequencies is in-phase/quadrature down-converted as a whole. A complete behavioral model for the total nonlinear distortion of the whole receiver chain is first derived, taking into account the third-order nonlinear distortion effects in all individual components, namely, RF low-noise amplifier, in-phase/quadrature mixer, and baseband in-phase/quadrature amplifiers. Stemming from this modeling, an adaptive digital feed-forward linearization structure is then developed to efficiently mitigate the joint nonlinear distortion of the whole receiver. The effectiveness of this approach is verified through extensive simulations and actual RF system measurements with a commercially available software-defined radio platform, which clearly outperforms the existing state-of-the-art methods that do not jointly consider RF and baseband nonlinearities.

A Class of Quantizers With DC-Free Quantization Noise and Optimal Immunity to Nonlinearity-Induced Spurious Tones

Fractional- N phase-locked loops (PLLs) typically use noise-shaping coarse quantizers to control their instantaneous output frequency. The resulting quantization noise and its running sum inevitably get distorted by non-ideal analog components within the PLL, which induces undesirable spurious tones in the PLL's output signal. A recently proposed quantizer, called a successive requantizer, has been shown to mitigate this problem. Its quantization noise and the running sum of its quantization noise can be subjected to up to fifth-order and third-order nonlinear distortion, respectively, without inducing spurious tones. This paper extends the previously published successive requantizer results to enable the design of successive requantizers whose quantization noise running sum sequences attain such immunity to nonlinearity-induced spurious tones up to arbitrarily high orders of distortion. The extended results are used to design example successive requantizers whose quantization noise and quantization noise running sum sequences have optimally reduced susceptibility to nonlinearity-induced spurious tones.

Linearized analog photonic links based on a dual-parallel polarization modulator

A linearized analog photonic link (APL) is proposed based on an integratable electro-optic dual-parallel polarization modulator (DPPolM), which consists of two polarization beam splitters and two polarization modulators (PolMs). Theoretical analysis shows that the APL is potentially free from the third-order nonlinear distortion if a polarization controller placed before the DPPolM is carefully adjusted. A proof-of-concept experiment is carried out. A reduction of the third-order intermodulation components as high as 40 dB and an improvement of the spurious-free dynamic range as large as 15.5 dB is achieved as compared with a single PolM-based link. The DPPolM-based APL is simple, compact, and power efficient since it requires only one laser, one modulator, and one photodetector.

New Low-Distortion Band-Switching Techniques for SAW Antenna Duplexers Used in Ultra-Wide-Band Cellular Phone

A new low-distortion band-switching technique for a surface-acoustic-wave (SAW) antenna duplexer used in ultra-wide-band cellular-phone systems has been developed. Both a transmitter (Tx) portion and a receiver (Rx) portion of the duplexer consist of not only wide-band SAW bandpass filters (BPFs), but also band-rejection SAW filters (BRFs) whose rejection bands can be shifted by switching circuits. The spurious-response sensitivity caused by the third-order nonlinear distortion of the switching circuits was dramatically improved by the new above configurations, i.e., the combination of BPFs and BRFs. The developed duplexer for the 900-MHz Japanese cdmaOne system has a size of 10 /spl times/ 8 /spl times/ 2 mm. It also has frequency characteristics of equivalent transmitted/received frequency bands as wide as 38 MHz and the guard band as narrow as 17 MHz. The insertion losses in both the low and high bands of the Tx portion are less than 2 dB, and those of the Rx portion are less than 3.3 dB. The other required specifications, e.g., attenuation at harmonic frequencies, attenuation at the image frequency, etc. are also satisfied.

Optical single-sideband modulator for broad-band subcarrier multiplexing systems

We experimentally demonstrate an enhanced optical single-sideband (OSSB) modulator for subcarrier-multiplexing (SCM) systems that provides improvement of the output optical modulation depth without increasing either the second-order or the third-order nonlinear distortion of the transmitted radio frequency signal. The modulator is deployed in an SCM link conveying 4 /spl times/ 622-Mb/s modulated subcarriers, demonstrating an enhancement of five orders of magnitude in bit error rate compared to the use of a dual-electrode OSSB modulator design. In addition, the dispersion penalty for the worst-case channel is reduced to less than 1.7 dB.

Linearization of 1.3-/spl mu/m MQW electroabsorption modulators using an all-optical frequency-insensitive technique

A novel all-optical linearization technique has been developed for discrete multiquantum-well electroabsorption modulators by exploiting the wavelength dependence of the quantum-confined Stark effect. The correcting signal for a particular order of nonlinearity is generated by the same modulator at a detuned optical wavelength, canceling the undesired order of nonlinear distortion via coherent RF recombination at the photodetector. This dual-wavelength approach is simple and insensitive to the microwave subcarrier frequency. A dramatic 30-dB reduction in the third-order nonlinear distortion at 10 GHz and 8-dB increase in the suboctave link dynamic range were achieved experimentally.

Golay sequences for identification of linear systems with weak nonlinear distortion

The design of bipolar excitation sequences for identifying linear time-invariant systems in the presence of weak nonlinear distortion is considered. A Volterra system model is used and system identification is carried out in the time domain by a crosscorrelation analysis. The measured system characteristic is the impulse response. A set of four pairs of Golay complementary sequences is presented which is very suitable for identification of linear systems suffering from weak second and third-order nonlinear distortion. Artefacts in the measured system characteristics caused by this kind of nonlinear distortion are effectively eliminated by averaging the four impulse responses obtained with the proposed four pairs of Golay complementary sequences. The key element in this study is higher-order autocorrelation properties of Golay complementary sequences. Experimental results obtained from a digital magnetic storage system are presented.

Dipulse-response measurement of a magnetic recording channel using Golay complementary sequences

We apply Golay complementary sequences for measuring the dipulse response and transfer functions of a digital magnetic-saturation-recording channel. Our experimental channel suffers from nonlinear distortion caused by a magnetoresistive (MR) sensor. The measured-channel characteristics are compared with the same characteristics obtained by the dipulse-response measurement technique by using feedback-shift register sequences. Based on a practical nonlinear Volterra model of a digital magnetic-saturation-recording channel, we analyze the artifacts caused by second- and third-order nonlinear distortion, which do-occur in the Golay dipulse responses and transfer functions. We show that with either of the considered dipulse-response measurement techniques, the artifacts caused by even-order nonlinear distortion, which occur in the measured-channel characteristics, can easily be removed. The measured-channel characteristics obtained show excellent performance for either of the considered excitation sequences.